Known as a method for non-destructive inspection of surface flaws or crack flaws in a cylindrical or columnar metallic material such as, for example, a steel bar or a steel pipe is an eddy current inspection method wherein a high frequency magnetic field of several KHz to several of tens of KHz is applied to a material to be examined so that the state of an eddy current generated in the surface of the examinee material is changed dependent on the presence or absence of a flaw; followed by a change in magnetic flux generated by the eddy current, and the change is detected by a detecting element such as a detection coil disposed in the proximity of the examinee material. In the case of the material to be examined being non-magnetic, a magnetic inspection method is known wherein a detecting element such as a Hall element or a detection coil disposed in the proximity of the material to be examined detects that the state of leakage magnetic flux generated at the surface of the examinee material by a high frequency magnetic field mentioned as above is changed dependent on the presence or absence of a flaw. Principles of these detection methods are disclosed in detail in "Industrial Instrumentation Handbook" published by Asakura Shoten, March, 1982, pp. 633-641.
When inspecting the entire circumference of the material to be examined in accordance with any of the above inspection methods, it is necessary to rotate the material to be examined while an inspection apparatus, comprised of an exciter for application of the magnetic field and the detecting element, is kept stationary; or it is necessary to rotate the inspection apparatus around the material to be examined while the examinee material is kept stationary. In either case, a rotation mechanism is required, therefore, increasing the overall size of the apparatus. Furthermore, the inspection is time-consuming. To solve these problems, a method is proposed in the specification of British Pat. No. 1,436,186, according to which a cylindrical exciter of a multiphase alternating current winding structure such as a stator of an induction motor is provided. A multiphase alternating current is passed through the exciter to generate a rotating magnetic field; and surface flaws of a material to be examined, which is inserted for passage through the exciter, are detected by a toroidal coil disposed in the proximity of the surface of the examinee material and surrounding the same. In this method, however, the multiphase alternating current to be passed through the exciter is of a single frequency as described in the aforementioned British Patent Specification; and this method is suitable for use with a low frequency. But if use with a high frequency, the core of the exciter mut be removed; alternatively the core must be made of a special material such as ferrite. When the low frequency magnetic field is used, sensitivity is degraded, and especially sensitivity to surface flaws of the material to be examined is seriously degraded and the detection becomes almost impossible. Especially where the high frequency is used with the core made of such a material as ferrite, an attendant increase in the number of revolutions of the rotating magnetic field gives rise to an increase in noise, thus preventing the frequency from increasing to an extent that practically sufficient sensitivity can be obtained.
Accordingly, an object of this invention is to provide a method and apparatus for non-destructive inspection of flaws in metal materials which does not require relative mechanical rotation between a material to be examined and the inspection apparatus and which can exhibit sufficient sensitivity of detection of surface flaws of the material to be examined.